CN105715452B

CN105715452B - System and method for increasing wind turbine power output

Info

Publication number: CN105715452B
Application number: CN201511036173.XA
Authority: CN
Inventors: T·埃登菲尔德
Original assignee: General Electric Co
Current assignee: General Electric Renovables Espana SL
Priority date: 2014-12-18
Filing date: 2015-12-18
Publication date: 2020-11-24
Anticipated expiration: 2035-12-18
Also published as: US9617976B2; CA2914293C; CA2914293A1; US20160369776A1; EP3034868A1; DK3034868T3; US9447772B2; ES2860601T3; CN105715452A; US20160177923A1; EP3034868B1

Abstract

Systems and methods for increasing the power output of wind turbines in a wind farm are disclosed. In particular, the wind farm can comprise first and second doubly fed induction generator wind turbine systems. The rotating rotor speed of the first wind turbine system is adjustable at the reduced wind speed based at least in part on the data indicative of the rotor voltage to increase the power output of the doubly-fed induction generator. The rotor speed can be adjusted such that the rotor voltage does not exceed the voltage threshold. The power output of the first wind turbine system can be further increased by reducing the reactive power output of the first wind turbine system. The reduced reactive power output of the first wind turbine system can be compensated by the increased reactive power output of the second wind turbine system.

Description

System and method for increasing wind turbine power output

Technical Field

The present disclosure relates generally to wind turbines, and more particularly to systems and methods for increasing the power output of doubly-fed induction generator wind turbine systems at reduced wind speeds.

Background

Wind power has received much attention as one of the cleanest, most environmentally friendly energy sources currently available. A typical modern wind turbine can include a tower, generator, gearbox, nacelle, and rotor with one or more rotor blades. The rotor blades are capable of converting wind energy into a mechanical rotational torque that drives one or more generators via the rotor. One or more generators can be coupled to the rotor, for example, via a gearbox. The gearbox is capable of stepping up the inherently low rotational speed of the rotor so that the generator is capable of efficiently converting mechanical rotational energy into electrical energy, which can be fed into a utility grid via at least one electrical connection.

Wind turbines can use variable speed operation such that the speed of the turbine blades changes with changes in wind speed. However, as the speed of the turbine fluctuates, the frequency of the alternating current flowing from the generator also fluctuates. Thus, the variable speed turbine configuration can also incorporate a power converter that can be used to convert the frequency of the generated electricity to a frequency substantially similar to the utility grid frequency. Such power converters can typically include an AC-DC-AC topology with a regulated DC link, and can be controlled by a converter controller.

Such wind turbines are capable of optimizing loads and improving turbine output using variable speed operation. In particular, they are most efficient when the wind turbine is operating at an optimal tip speed ratio. The tip speed ratio is the ratio between the tangential speed of the tip of the turbine blade and the speed of the wind at the wind turbine. Thus, a wind turbine operating at an optimal tip speed ratio will collect more wind energy than it would if operating outside of the optimal tip speed ratio.

For many wind turbines, the operating space, and thus the number, of consumers (customers) is limited by the maximum voltage of one or more wind turbine components inherent to the wind turbine system. For example, a power converter in a wind turbine system can have voltage constraints that limit the minimum and maximum speed values of the generator.

Accordingly, there is a need for a system and method for increasing the power output of a wind turbine system at reduced wind speeds while also maintaining the power converter voltage level within specified operating limits.

Disclosure of Invention

Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.

One example aspect of the present disclosure is directed to a wind turbine system. A wind turbine system includes a wind driven doubly fed induction generator having a rotor and a stator. The stator provides AC power to the stator bus. The wind turbine system further comprises a power converter coupled to the rotor of the doubly-fed induction generator via a rotor bus. The power converter provides an output to the line bus. The power converter has an associated voltage threshold at the rotor bus. The wind turbine system also includes a control system configured to identify a reduced wind speed at the doubly fed induction generator. At the reduced wind speed, the control system is further configured to adjust a rotational speed of a rotor of the doubly-fed induction generator based at least in part on the data indicative of the rotor voltage to increase a power output of the doubly-fed induction generator. The control system adjusts the rotational speed of the rotor such that the rotor voltage does not exceed a voltage threshold associated with the power converter at the rotor bus.

Another example aspect of the present disclosure is directed to a method for increasing the power output of a wind driven doubly fed induction generator at reduced wind speeds. The method includes generating ac power at a wind-driven doubly-fed induction generator. Alternating current power is supplied from the stator of the wind driven doubly fed induction generator to the stator bus. The method also includes providing the rotor voltage from the power converter to a rotor of the wind-driven doubly-fed induction generator via a rotor bus. The method also includes detecting a reduced wind speed at the wind-driven doubly-fed induction generator. The method also includes, in response to detecting the reduced wind speed, reducing a rotational speed of the rotor from a first rotational speed to a second rotational speed based at least in part on the data indicative of the rotor voltage to increase a power output of the doubly-fed induction generator. The second rotational speed is determined such that the rotor voltage does not exceed a voltage threshold associated with the power converter at the rotor bus.

Yet another example aspect of the present disclosure is directed to a wind farm. The wind farm includes a first doubly fed induction generator having a rotor and a stator, a second doubly fed induction generator having a rotor and a stator, and a control system. The control system is configured to detect a reduced wind speed at the first double fed induction generator. In response to detecting a reduced wind speed at the first fed induction generator, the control system is configured to control the first fed induction generator to reduce the reactive power output of the first fed induction generator and to reduce the rotational speed of the rotor of the first fed induction generator to increase the power output of the first fed induction generator. The control system is further configured to control the second doubly fed induction generator to increase the reactive power output of the second doubly fed induction generator.

Variations and modifications can be made to these example aspects of the disclosure.

These and other features, aspects, and advantages of various embodiments will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the description, serve to explain the relevant principles.

The invention provides the following technical scheme:

1. a wind turbine system, comprising:

a wind driven doubly fed induction generator having a rotor and a stator, said stator providing AC power to stator bus bars;

a power converter coupled to the rotor of the doubly-fed induction generator via a rotor bus, the power converter providing an output to a line bus, the power converter having an associated voltage threshold at the rotor bus; and

a control system configured to identify a reduced wind speed at the doubly-fed induction generator;

wherein at the reduced wind speed, the control system is further configured to adjust a rotational speed of the rotor of the doubly-fed induction generator to increase a power output of the doubly-fed induction generator based at least in part on data indicative of a rotor voltage, the control system adjusting the rotational speed of the rotor such that the rotor voltage does not exceed a voltage threshold associated with the power converter at the rotor bus.

2. The wind turbine system of claim 1, wherein the reduction in the rotational speed of the rotor of the doubly-fed induction generator provides increased slip at the doubly-fed induction generator.

3. The wind turbine system of claim 1, wherein at the reduced wind speed, the control system is configured to adjust the rotational speed of the rotor of the doubly-fed induction generator based at least in part on a tip speed ratio associated with the wind turbine, the tip speed ratio being a ratio between a tangential speed of a tip of a blade of the wind turbine and the wind speed.

4. The wind turbine system of claim 1, wherein the data indicative of the rotor voltage is determinable based at least in part on one or more sensors configured to monitor a voltage at the rotor bus.

5. The wind turbine system of claim 1, wherein the data indicative of the rotor voltage is determinable based at least in part on a reactive power output of the doubly-fed induction generator.

6. The wind turbine system of claim 1, wherein the data indicative of the rotor voltage comprises a look-up table defining correlations between various rotor speed points and various grid voltage and reactive power demand conditions, the reduced rotational speed of the rotor of the doubly-fed induction generator being determined based at least in part on the look-up table.

7. The wind turbine system of claim 1, wherein the control system is configured to adjust the rotational speed based at least in part on a grid voltage.

8. The wind turbine system of claim 1, wherein at the reduced wind speed, the control system is configured to reduce the reactive power output of the doubly-fed induction generator to allow an incremental reduction in the rotational speed of the rotor of the doubly-fed induction generator.

9. A method for increasing the power output of a wind driven doubly fed induction generator at reduced wind speeds, the method comprising:

generating AC power at a wind driven doubly fed induction generator, the AC power being provided from a stator of the wind driven doubly fed induction generator to a stator bus;

providing a rotor voltage from a power converter to a rotor of the wind driven doubly fed induction generator via a rotor bus;

detecting a reduced wind speed at the wind driven doubly fed induction generator; and

in response to detecting the reduced wind speed, reducing a rotational speed of the rotor from a first rotational speed to a second rotational speed to increase the power output of the doubly-fed induction generator based at least in part on data indicative of the rotor voltage, the second rotational speed determined such that the rotor voltage does not exceed a voltage threshold associated with the power converter at the rotor bus.

10. The method of claim 9, wherein the reduced rotational speed of the rotor of the doubly-fed induction generator is determined based at least in part on a tip speed ratio, the tip speed ratio being a ratio between a tangential speed of a tip of a blade of the wind turbine and the wind speed.

11. The method of claim 9, wherein the data indicative of the rotor voltage is determinable based at least in part on one or more sensors configured to monitor a voltage at the rotor bus.

12. The method of claim 9, wherein the data indicative of the rotor voltage is determinable based at least in part on a reactive power output of the doubly fed induction generator.

13. The method of claim 9, wherein the method comprises:

reducing the reactive power output of the wind-driven doubly-fed induction generator; and

reducing the rotational speed of the rotor of the wind driven doubly fed induction generator to a third rotational speed, the third rotational speed being less than the first rotational speed.

14. The method of claim 9, wherein reducing the second rotational speed of the rotor of the doubly fed induction generator provides increased slip of the doubly fed induction generator.

15. A wind farm, the wind farm comprising:

a first wind turbine system having a first double fed induction generator having a rotor and a stator;

a second wind turbine system having a second doubly fed induction generator, the second doubly fed induction generator having a rotor and a stator; and

a control system configured to detect a reduced wind speed at the first double fed induction generator;

wherein in response to detecting the reduced wind speed at the first fed induction generator, the control system is configured to control the first fed induction generator to reduce a reactive power output of the first fed induction generator and to reduce the rotational speed of the rotor of the first fed induction generator to increase a power output of the first fed induction generator,

wherein the control system is further configured to control the second doubly fed induction generator to increase the reactive power output of the second doubly fed induction generator.

16. The wind farm of claim 15, wherein the rotor of the first doubly fed induction generator is coupled via a rotor bus to a power converter having an associated voltage threshold at the rotor bus.

17. The wind farm of claim 16, wherein the rotational speed of the rotor of the first double fed induction generator is reduced such that the power converter voltage at the rotor bus does not exceed the voltage threshold.

18. The wind farm of claim 15, wherein the increased reactive power output of the second doubly fed induction generator is determined based at least in part on a reactive power demand from a grid.

19. The wind farm of claim 15, wherein the reduction in the rotational speed of the rotor of the first doubly fed induction generator provides increased slip at the doubly fed induction generator.

20. The wind farm of claim 15, wherein the reduced reactive power output of the first doubly-fed induction generator facilitates an incremental reduction in rotational speed of the rotor of the doubly-fed induction generator such that a power converter voltage at the rotor bus does not exceed the voltage threshold.

Drawings

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:

FIG. 1 depicts an example doubly-fed induction generator (DFIG) wind turbine system, according to an example embodiment of the present disclosure;

FIG. 2 depicts an example controller according to an example embodiment of the present disclosure;

FIG. 3 depicts a flowchart of an example method for increasing power output of a DFIG wind turbine system, according to an example embodiment of the present disclosure;

FIG. 4 depicts an example wind farm, according to an example embodiment of the present disclosure;

FIG. 5 depicts a flowchart of an example method for increasing power output of a wind farm, according to an example embodiment of the present disclosure.

Detailed Description

Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. It is therefore intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents.

Example aspects of the present disclosure are directed to systems and methods for increasing power output in a doubly-fed induction generator (DFIG) wind turbine system at reduced wind speeds. The DFIG system can include a wind driven doubly fed induction generator having a rotor and a stator. The stator is capable of providing AC power to the stator bus. The rotor is capable of providing AC power to the power converter via the rotor bus. The power converter can provide an output to the line bus. The stator bus and the line bus can be coupled to the grid by a transformer, such as a two-winding transformer or a three-winding transformer.

According to an example aspect of the present disclosure, the rotational speed of the rotor of the doubly-fed induction generator can be adjusted at reduced wind speeds to increase the output power of the doubly-fed induction generator system. For example, the rotational speed of the rotor can be reduced to operate the wind turbine at or near the optimal tip speed ratio at reduced wind speeds.

Reducing the rotor speed at the cut-in wind speed can provide increased slip at the doubly fed induction generator. In particular, slip is the difference between the synchronous speed and the operating speed of a doubly fed induction generator divided by the synchronous speed. The operating speed is the rotational speed of the rotor and the synchronous speed is the rotational speed of the magnetic field of the stator. Increasing slip can cause an increased rotor voltage of the doubly fed induction generator. The rotor voltage is equal to the locked rotor voltage multiplied by the slip. Thus, a reduction in the rotating rotor speed of the doubly fed induction generator can provide an increased slip, which results in an increased rotor voltage.

The minimum and maximum speed points of a doubly fed induction generator system are generally limited by the voltage capability of the power converter. Other factors can include grid voltage and reactive power requirements. Thus, at cut-in wind speeds, the rotational speed of the rotor of the doubly-fed induction generator can be adjusted such that the power converter voltage at the rotor bus does not exceed the voltage capability of the power converter. This adjustment can improve wind turbine efficiency by increasing the speed range of the rotor at the cut-in wind speed and causing the wind turbine system to harvest more energy from the wind.

More particularly, the rotational speed of the rotor can be adjusted in accordance with example aspects of the present disclosure based at least in part on the data indicative of the rotor voltage to increase the power output of the doubly-fed induction generator at reduced wind speeds. For example, the rotational speed of the rotor can be adjusted such that the rotor voltage does not exceed a voltage threshold associated with the power converter at the rotor bus. The data indicative of the rotor voltage can be determined, for example, by one or more sensors. For example, one or more sensors can be placed at the rotor bus to detect the voltage at the rotor of the doubly fed induction generator. This detected rotor voltage can be used by a controller (e.g., a wind farm control system and/or an individual wind turbine controller) to adjust the rotational speed of the rotor such that the rotor voltage does not exceed a voltage threshold.

The data indicative of the rotor voltage can also be determined in other suitable ways, such as from the reactive power output of the doubly fed induction generator. For example, the look-up table can be used by the controller to adjust the rotating rotor speed of the doubly fed induction generator. The look-up table can define correlations between various rotor speed points and various grid voltage and reactive power/power factor conditions.

According to an example aspect of the present disclosure, the rotor speed range can be further increased by reducing the reactive power output of the doubly fed induction generator at reduced wind speeds. This reduction in reactive power output can be used to allow the doubly fed induction generator wind turbine system to operate at or near the optimal tip speed ratio. For example, if the rotor voltage is at a voltage threshold but the wind turbine system is not operating at the optimal tip speed ratio, a further reduction in the rotating rotor speed may be necessary for the wind turbine system to operate at or near the optimal tip speed ratio. Therefore, the reactive power output of the doubly fed induction generator can be reduced to facilitate such a reduction in the rotating rotor speed.

The rotor voltage can depend on the grid voltage and the reactive power demand. Thus, reducing the reactive power output at reduced windage can allow for an enhanced reduction in the rotational speed of the rotor of the doubly-fed induction generator without exceeding the voltage capability of the power converter. The increased reduction in the rotational speed of the rotor can be such that the power converter voltage at the rotor bus does not exceed a voltage threshold.

In one example implementation, a wind farm can include a plurality of wind turbines, such as a first wind turbine system and a second wind turbine system, each coupled to a power grid. The wind farm can also include a control system configured to detect a reduced wind speed at the doubly fed induction generator of the first wind turbine system. For example, the first wind turbine system can be located in the middle of the wind farm, where the wind speed can sometimes be reduced relative to the wind speed at the periphery of the wind farm. In response to detecting a reduced wind speed, the control system can be configured to control the doubly-fed induction generator of the first wind turbine system to reduce the reactive power output of the first wind turbine system. This can allow for an enhanced reduction in the rotational speed of the rotor of the doubly-fed induction generator associated with the first wind turbine system. The rotational speed of the rotor can be reduced such that the voltage of a power converter coupled to the doubly-fed induction generator via the rotor bus does not exceed a voltage threshold at the rotor bus.

The control system can be further configured to control the doubly-fed induction generator of the second wind turbine system to increase the reactive power output of the doubly-fed induction generator. This increase in reactive power output can be determined based at least in part on the reactive power demand from the grid. In particular, the increased reactive power output of the second wind turbine system can compensate for the reduced reactive power output of the first wind turbine system, e.g. to meet the reactive power demand of the grid.

Referring now to the drawings, FIG. 1 depicts an example doubly-fed induction generator (DFIG) wind turbine system 100, according to an example embodiment of the present disclosure. The system 100 includes a plurality of rotor blades 108 coupled to a rotating shaft 110, which collectively define a propeller 106. The propeller 106 is coupled to an operating gear box 118, which in turn is coupled to a generator 120. According to aspects of the present disclosure, the generator 120 is a doubly-fed induction generator (DFIG) 120.

DFIG 120 is generally coupled to stator 154 and power converter 162 via rotor bus 156. Stator bus 154 provides output multi-phase power (e.g., three-phase power) from the stator of DFIG 120, and rotor bus 156 provides output multi-phase power (e.g., three-phase power) from the rotor of DFIG 120. The DFIG 120 can also be coupled to a controller 174 to control operation of the DFIG 120. It should be noted that controller 174 is configured in the exemplary embodiment as an interface between DFIG 120 and control system 176. The controller 174 can include any number of control devices. In one implementation, the controller 174 can comprise a processing device (e.g., a microprocessor, microcontroller, etc.) that executes computer-readable instructions stored in a computer-readable medium. The instructions, when executed by the processing device, are capable of causing the processing device to perform operations including providing control commands to the DFIG 120.

For example, as particularly shown in fig. 2, the controller 174 can include any number of control devices. In one implementation, for example, the controller 174 can include one or more processors 190 and associated memory devices 192 configured to perform various computer-implemented functions and/or instructions (e.g., to perform methods, steps, calculations, etc., and to store related data as disclosed herein). The instructions, when executed by the processor 190, can cause the processor 190 to perform operations including providing control commands (e.g., pulse width modulation commands) to switching elements of the power converter 162 and other aspects of the power system 100. Accordingly, controller 174 may also include a communications module 194 that facilitates communication between control system 174 and any of the various components of power system 100, such as the components of fig. 1.

Further, communication module 194 may include a sensor interface 196 (e.g., one or more analog-to-digital converters) that permits signals communicated from one or more sensors to be transformed into signals capable of being processed and understood by processor 190. It is to be appreciated that sensors (e.g.,

sensors

191, 193, 195) can be communicatively coupled to communication module 194 using any suitable means. For example, as shown in FIG. 2,

sensors

191, 193, 195 are coupled to sensor interface 196 via a wired connection. However, in other embodiments, the

sensors

191, 193, 195 may be coupled to the sensor interface 196 via a wireless connection, such as by using any suitable wireless communication protocol known in the art. Likewise, processor 190 may be configured to receive one or more signals from the sensors.

As used herein, the term "processor" refers not only to integrated circuits as is referred to in the art as being included in a computer, but also to controllers, microcontrollers, microcomputers, Programmable Logic Controllers (PLCs), application specific integrated circuits, and other programmable circuits. The processor 190 is also configured to compute advanced control algorithms and communicate various ethernet or serial-based protocols (Modbus, OPC, CAN, etc.). Additionally, the one or more memory devices 192 may generally include one or more memory elements including, but not limited to, a computer-readable medium (e.g., Random Access Memory (RAM)), a computer-readable non-volatile medium (e.g., flash memory), a floppy diskette, a compact disk read-only memory (CD-ROM), a magneto-optical disk (MOD), a Digital Versatile Disk (DVD), and/or other suitable memory elements. Memory device(s) 192 may generally be configured to store suitable computer-readable instructions that, when implemented by one or more processors 190, configure controller 174 to perform various functions as described herein.

Referring back to FIG. 1, DFIG 120 is coupled to rotor side converter 166 via rotor bus 156. The rotor side converter 166 is coupled to a line side converter 168, which is in turn coupled to a line side bus 188. In an example configuration, the rotor-side converter 166 and the line-side converter 188 are configured using Insulated Gate Bipolar Transistor (IGBT) switching elements for a normal mode of operation in a three-phase, Pulse Width Modulation (PWM) arrangement. The rotor side converter 166 and the line side converter 168 can be coupled via the DC link 136, across the DC link 136 is a DC link capacitance 138.

The power converter 162 can also be coupled to a controller 174 to control the operation of the rotor side converter 166 and the line side converter 168. In one implementation, other instructions stored in the controller 174, when executed by a processing device, can cause the processor to perform operations including providing control commands (e.g., pulse width modulation commands) to the switching elements of the power converter 162.

In a typical configuration, various circuit breakers including, for example, grid circuit breaker 182 and line contactors can include various components for isolating during connection to and disconnection from grid 184 as necessary for normal operation of DFIG 120. System breaker 178 can couple system bus 160 to transformer 180, which is coupled to grid 184 via grid breaker 182. Although transformer 180 depicts a two-winding transformer, other suitable transformers, such as a three-winding transformer, can be used. In embodiments using a three-winding transformer, the line bus 188 can be coupled to one winding of the transformer, the stator bus 154 can be coupled to another winding of the transformer, and the mesh 184 can be coupled to another winding of the transformer.

In operation, AC power generated by rotating rotor 106 at DFIG 120 is provided to grid 184 via dual paths. The dual path is defined by a stator bus 154 and a rotor bus 156. On rotor bus side 156, sinusoidal multiphase (e.g., three-phase) Alternating Current (AC) power is provided to power converter 162. Rotor-side power converter 166 converts the AC power provided from rotor bus 156 to Direct Current (DC) power and provides the DC power to DC link 136. Switching elements (e.g., IGBTs) used in the parallel bridge circuit of rotor-side power converter 166 can be modulated to convert AC power provided from rotor bus 156 to DC power suitable for DC link 136.

Line-side converter 168 converts the DC power on DC link 136 to AC output power suitable for use by grid 184. In particular, the switching elements (e.g., IGBTs) used in the bridge circuit of the line-side power converter 168 can be modulated to convert the DC power on the DC link 136 to AC power on the line-side bus 188. The AC power from power converter 162 can be combined with power from the stator of DFIG 120 to provide multi-phase power (e.g., three-phase power) having a frequency substantially maintained at the frequency of grid 184 (e.g., 50Hz/60 Hz).

Various circuit breakers and switches, such as grid breaker 182, system breaker 178, stator synchronizing switch 158, converter breaker 186, and line contactor 172, can be included in the system 100 to connect or disconnect corresponding buses, for example, when current is excessive and can damage components of the wind turbine system 100 or for other operational considerations. Additional protection components can also be included in the wind turbine system 100.

The DFIG 120 and the power converter 162 are capable of receiving control signals from, for example, a control system 176 via a controller 174. The control signal can be based, among other things, on a sensed condition or operational characteristic of wind turbine system 100. Generally, the control signals provide control for operation of DFIG 120 and/or power converter 162. For example, feedback in the form of a voltage at rotor bus 156 of power converter 162 can be used to regulate the rotational speed of the rotor of DFIG 120. As another example, feedback in the form of sensed rotor speed of DFIG 120 can be used to control conversion of output power from rotor bus 156 to maintain a proper and balanced multi-phase (e.g., three-phase) power source. Other feedback from other sensors can also be used by controller 174 to control DFIG 120 and/or power converter 162, including, for example, stator and rotor bus voltage and current feedback. Using various forms of feedback information, switching control signals (e.g., gate timing commands for IGBTs), stator synchronization control signals, and breaker signals can be generated.

According to aspects of the present disclosure, a rotational speed of a rotor of the DFIG 120 can be adjusted based at least in part on a voltage threshold associated with the power converter 166 at the rotor bus 156. In particular, the wind speed range of the system 100 can be limited by the voltage capability of the power converter 162. In particular, the minimum and maximum wind speed points of the system 100 can be limited by the voltage capability of the power converter 162. The voltage threshold associated with the power converter 162 can be determined based at least in part on the voltage capability of the power converter 162. Thus, at the cut-in wind speed, controller 174 can be configured to control DFIG 120 to reduce a rotational speed of a rotor of DFIG 120 such that a voltage of power converter 162 at rotor bus 156 does not exceed a voltage threshold associated with power converter 162.

For example, for a 1800V IGBT converter, the voltage capability can be in the range of approximately 759V. As used herein, the use of the term "about" in connection with a numerical value is intended to mean within about 25% of the stated numerical value. This voltage constraint can cause the system 100 to lose energy at the cut-in wind speed. Thus, at a power factor of 1, the rotational speed of the rotor of the 50Hz DFIG can be reduced to 925rpm at the cut-in wind speed as opposed to 1080rpm at which the DFIG will operate normally.

FIG. 3 depicts a flowchart of an example method (300) for increasing power output of a DFIG wind turbine system, according to an example embodiment of the present disclosure. For purposes of illustration and discussion, FIG. 3 depicts steps performed in a particular order. Those of ordinary skill in the art, using the disclosure provided herein, will appreciate that the various steps of any of the methods disclosed herein can be adjusted, omitted, rearranged or expanded in various ways without departing from the scope of the present disclosure.

At (302), the method (300) can include generating ac power at a wind-driven doubly-fed induction generator. Ac power can be provided from the stator of the wind driven doubly fed induction generator to the stator bus. At (304), the method (300) can include providing a rotor voltage from a power converter to a rotor of the wind-driven doubly-fed induction generator via a rotor bus.

At (306), the method (300) can include monitoring wind speed at the wind-driven doubly-fed induction generator. The wind speed can be determined, for example, by an anemometer associated with the wind driven doubly fed induction generator.

At (308), the method (300) can include detecting a reduced wind speed at the wind-driven doubly-fed induction generator. The reduced wind speed can be, for example, the cut-in wind speed of a wind driven doubly fed induction generator.

In response to detecting the reduced wind speed, at (310), the method (300) can include controlling a rotational speed of a rotor of the wind-driven doubly-fed induction generator based at least in part on the data indicative of the rotor voltage. The data indicative of the rotor voltage can include signals from one or more sensors at the rotor bus and/or data in a look-up table relating various rotor speed points to various grid voltage and reactive power/power factor conditions.

According to certain aspects of the present disclosure, the rotating rotor speed can be reduced such that the rotor voltage does not exceed a voltage threshold associated with the power converter at the rotor bus. For example, the controller can send a control command to the wind driven doubly fed induction generator to reduce the rotational rotor speed of the wind driven doubly fed induction generator from a first rotational speed to a second rotational speed. The second rotational speed can be determined such that the wind turbine system operates, for example, at or near the optimal tip speed ratio to increase the power output of the doubly fed induction generator. The second rotational speed can also be determined such that the power converter voltage at the rotor bus does not exceed the voltage threshold.

At (312), the method (300) can further include controlling a rotational speed of a rotor of the wind-driven doubly-fed induction generator based at least in part on a voltage threshold associated with the power converter. If the power converter voltage on the rotor side exceeds the voltage threshold, (312) can include increasing the rotational speed of the rotor to reduce the rotor voltage and slip of the doubly fed induction generator. The rotating rotor speed can be increased such that the power converter voltage at the rotor bus does not exceed the voltage threshold. If the voltage does not exceed the voltage threshold, (312) can include maintaining a rotational speed of a rotor of the DFIG, and the method (300) can include returning to (306).

According to an example aspect of the disclosure, a plurality of wind turbine systems, such as wind turbine system 100 depicted in FIG. 1, can be part of a wind farm. FIG. 4 depicts an example wind farm 200, according to an example embodiment of the present disclosure. Wind farm 200 includes a wind turbine system 202 and a wind turbine system 204.

Wind turbine systems

202 and 204 can be a DFIG system such as system 100 described in FIG. 1. Although only two wind turbine systems are depicted, it will be appreciated by those skilled in the art that any suitable number of wind turbine systems can be included in wind farm 200.

Wind turbine systems

202 and 204 can each be coupled to a wind farm controller 206. Wind farm controller 206 is configured in the exemplary embodiment as an interface between wind farm 200 and control system 208. Wind farm controller 206 can include any number of control devices. In one implementation, the wind farm control system can include a processor (e.g., a microprocessor, microcontroller, etc.) that executes computer-readable instructions stored in a computer-readable medium. The instructions, when executed by the processing device, can cause the processing device to perform operations including providing control commands to

wind turbine systems

202 and 204.

The wind farm 200 can also be coupled to a grid 210 via a transformer 212. The transformer 212 comprises a two-winding transformer, but it will be appreciated by those skilled in the art that various other suitable transformers can be used, such as a three-winding transformer. Wind farm 200 is capable of outputting multi-phase power (e.g., three-phase power) to a grid 210 via a transformer 212. The output power level of wind farm 200 can be controlled, at least in part, by wind farm controller 206.

In particular, wind farm controller 206 can receive a command value indicative of a target power value for wind farm 200 from, for example, control system 208. The target power value can comprise an active power value and/or a reactive power value. The target power value can be determined based at least in part on the power demand from the power grid 210. Further, the wind farm controller 206 can receive measurement data indicative of active power, reactive power, voltage, frequency, etc. of the wind farm 200 at the grid connection point.

Further still, wind farm controller 206 can receive turbine measurement data from

wind turbine systems

202 and 204. For example, the turbine measurement data can include frequency, voltage, current, active power output, reactive power output, wind speed, power factor, etc. of the power output from the DFIG. Based at least in part on the various received data, wind farm controller 206 can determine command values for

wind turbine systems

202 and 204.

Wind turbine systems

202 and 204 are then able to control respective

wind turbine systems

202 and 204 using a control system included with

wind turbine systems

202 and 204, such as control system 176 depicted in FIG. 1, in accordance with the command values from wind farm controller 206.

In the exemplary embodiment, wind farm controller 206 is capable of determining commanded control of

wind turbine systems

202 and 204 based at least in part on a lookup table. The look-up table can define correlations between various rotor speed points and various grid voltage and reactive power demand conditions.

FIG. 5 depicts a flowchart of an example method (400) for increasing power output of a wind farm at reduced wind speeds, according to an example embodiment of the present disclosure. The rotor speed range of a doubly fed induction generator can be further increased by reducing the reactive power output at reduced wind speeds. Accordingly, at (402), the method 400 can include identifying a reactive power demand of the grid. The grid can be coupled to a wind farm, such as wind farm 200 depicted in FIG. 4. The wind farm can comprise, for example, a first and a second double fed induction generator. The reactive power demand of the grid can be determined based at least in part on various grid conditions associated with electrical loads coupled to the grid.

At (404), the method (400) can include detecting a reduced wind speed at the first double-fed induction generator. The reduced wind speed can for example be the cut-in wind speed of a doubly fed induction generator. In response to detecting a reduced wind speed at the first doubly fed induction generator, at (406), the method (400) can include reducing a rotating rotor speed and a reactive power output of the doubly fed induction generator to increase a power output of the doubly fed induction generator. The rotational speed can be reduced such that the voltage of a power converter coupled to the rotor of the doubly-fed induction generator does not exceed a voltage threshold at the rotor bus.

Reducing the reactive power output can allow for an enhanced reduction in the rotational speed of the doubly fed induction generator. The enhanced reduction in the rotational speed of the rotor can be such that the voltage at the rotor bus of the power converter does not exceed a voltage threshold associated with the power converter.

At (408), the method 400 can include increasing the reactive power output of the second doubly fed induction generator. The reactive power output of the second doubly fed induction generator can be determined based at least in part on the identified grid reactive power demand. For example, the reactive power output of the second fed induction generator can be increased to compensate for the reduced reactive power output of the first fed induction generator. The increased reactive power output of the second doubly fed induction generator can be determined such that the total reactive power output of the wind farm meets the required reactive power demand of the wind farm from the grid.

Examples of the invention

Table 1 shows example simulation results according to example embodiments of the present disclosure. In particular, table 1 shows the power output of the doubly fed induction generator at reduced wind speeds for different rotating rotor speeds. For example, as shown by Table 1, at a wind speed of 4m/s, reducing the rotating rotor speed from 1065rpm to 925rpm provides a 31kW increase in the power output of the doubly fed induction generator.

	1065rpm	925rpm	Difference (D)
				Wind speed [ m/s ]]	Power [ kW ]]	Power [ kW ]]	Power [ kW ]]
3.00	5	29	24
				3.50	63	90	17
4.00	129	160	31
				4.50	225	239	14
5.00	301	328	27
				5.50	470	474	4
6.00	589	597	8

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

The following is a list of components herein.

Reference symbol assembly

100 wind turbine system

106 propeller

108 multiple rotor blades

110 rotating shaft

118 gear box

120 double-fed induction generator (DFIG)

136 DC link

138 DC Link capacitor

154 stator bus

156 rotor bus

158 stator synchronous switch

160 system bus

162 power converter

166 rotor side converter

168 line side converter

172 line connector

174 controller

176 control system

178 System Circuit breaker

180 transformer

182 grid circuit breaker

184 electric network

186 converter circuit breaker

188 line side bus

190 one or more processors

191 sensor

192 one or more memory devices

193 sensor

194 communication module

195 sensor

196 sensor interface

200 wind farm

202 wind turbine system

204 wind turbine system

206 wind farm controller

208 control system

210 grid

212 Transformer

300 method

302 method step

304 method step

306 method step

308 method step

310 method step

312 method step

400 method

402 method step

404 method step

406 method step

408 method step

Claims

1. A wind turbine system, comprising:

3. The wind turbine system of claim 1, wherein at the reduced wind speed, the control system is configured to adjust the rotational speed of the rotor of the doubly-fed induction generator based at least in part on a tip speed ratio associated with the wind turbine, the tip speed ratio being a ratio between a tangential speed of a tip of a blade of the wind turbine and the reduced wind speed.

10. The method of claim 9, wherein the reduced rotational speed of the rotor of the doubly-fed induction generator is determined based at least in part on a tip speed ratio, the tip speed ratio being a ratio between a tangential speed of a tip of a blade of a wind turbine and the reduced wind speed.

12. The method of claim 9, wherein the data indicative of the rotor voltage is determinable based at least in part on a reactive power output of the doubly-fed induction generator.

13. The method of claim 9, wherein the method comprises:

14. The method of claim 9, wherein reducing the second rotational speed of the rotor of the doubly-fed induction generator provides increased slip of the doubly-fed induction generator.

15. A wind farm, the wind farm comprising:

wherein in response to detecting the reduced wind speed at the first doubly-fed induction generator, the control system is configured to control the first doubly-fed induction generator to reduce a reactive power output of the first doubly-fed induction generator and to reduce a rotational speed of the rotor of the first doubly-fed induction generator to increase a power output of the first doubly-fed induction generator,

16. The wind farm of claim 15, wherein the rotor of the first double fed induction generator is coupled to a power converter via a rotor bus, the power converter having an associated voltage threshold at the rotor bus.

17. The wind farm of claim 16, wherein the rotational speed of the rotor of the first double fed induction generator is reduced such that a power converter voltage at the rotor bus does not exceed the voltage threshold.

19. The wind farm of claim 15, wherein a decrease in the rotational speed of the rotor of the first doubly fed induction generator provides increased slip at the doubly fed induction generator.

20. The wind farm of claim 16, wherein the reduced reactive power output of the first doubly-fed induction generator facilitates an incremental reduction in rotational speed of the rotor of the doubly-fed induction generator such that a power converter voltage at the rotor bus does not exceed the voltage threshold.